Charging for dogs. Why?

Can anyone give me a reasonable explanation as to why many sites charge extra if you have a dog with you?  Here's my take on the matter.  Suppose 10% of caravans on a 100 pitch site have a dog (I suspect the percentage is higher than that but let's not quibble).  And suppose they charge £1.50 per dog per day.  Who gets that £15 and what for?  It can't be for picking up dog mess as most people do that anyway.  Even if there is the occasional mess left, it can't take £15 of work for a warden to go and dispose of it.  As for the dog walk itself, does it require £15 of daily maintenance?  On some big sites I suspect that the amount generated by charging for dogs is much higher, perhaps even a three figure sum per day.  We are being asked to pay for our dogs and being offered little or nothing in return.  This complaint isn't directed at CC by the way, it's more against commercial sites.  Any thoughts?